Whole IT companies are built on Microsoft backbone, from enterprise sw, security sw, client sw, everything enterprise SQL, windows server, sharepoint, exchange, lync, forefront, system centre and so on and so forth. They are all built to serve the business, to serve the users, the client users all interact in the back end with all these applications and the biggest IT issue out there today apart from virtualisation and public/private cloud technology solutions is client mobility. The ability to bring your device anywhere with you and have access to the exact same apps, setup, desktop etc with the exac same service. Work from your car or on th beach exactly the same as you would in the office. It's easier for this to happen if the client sw is windows, well it is in Microsoft eyes.
So why not create the hardware as well? To me it really fucks off the OEM partners Microsoft already has but I suppose my point is, you can't do all the above with too 100% efficiency with an iPad. How many IT deps are built on iOS? Not many I would guess!